T lymphocytes are important effector cells in natural antiviral and anticancer immunity. We explored the hypothesis that the final outcome of T cell antigen receptor-driven immune processes is at least partially determined by physiologically abundant small signaling molecules in the extracellular environment of lymphocytes in different tissues. For historical reasons, the extracellular purines (ATP and adenosine) and their (purinergic) receptors were studied as an example of such molecules. The results of our studies indicate that A2a adenosine receptors (A2aR) on the T cell surface may play an immunosuppressive role under conditions that lead to accumulation of extracellular adenosine (extAdo). Extracellular adenosine was found in our studies to be able to induce apoptosis in about 10% to 15% of thymocytes on its own and to protect virtually all thymocytes from antigen receptor (TCR)-induced apoptosis. These properties suggest that extAdo and receptors for extAdo may serve as immunoregulators. To evaluate the mechanism of adenosine action on lymphocytes we used A2aR gene-deficient (A2aR -/-) mice, since the A2aRs were shown to be predominantly expressed in thymocytes in studies with selective agonists and antagonists of A2aR and cDNA probes. It was shown that thymocytes from A2aR -/- mice were resistant to adenosine-induced apoptosis, providing genetic evidence for a signaling mechanism of adenosine-induced apoptosis and for thymocyte subset-specific expression of A2aRs in thymocytes. Thus, the direct apoptotic effects of exogeneous adenosine are completely accounted for by signaling through A2aR with no contribution of intracellular lymphotoxicity or compensating A2b receptors. Unexpectedly, both wild-type and A2aR- deficient thymocytes were ?rescued? by extAdo from TCR-induced apoptosis, suggesting that while only about 10% of thymocytes expresses apoptosis-inducing A2aRs, almost all of them may express an as-yet- unidentified TCR signal-inhibiting adenosine receptor. These data raise questions about the identity of A2aR-expressing thymocytes, the possible functional role of A2a receptors in thymocyte differentiation, and the contribution of a novel, as-yet-unidentified receptor for extAdo in the effects of extAdo on T cells. - Antigen receptors, extracellular matrix, lymphocyte development, ATP, adenosine, immunodeficiency, immunopathology